The present preferred embodiment relates to superconducting electromagnets comprising coils of superconducting wire bonded to a support structure.
In particular, the preferred embodiment relates to improvements in such assemblies which reduce thermally-induced stresses between the coils and the support structure in cases of abrupt temperature changes to the assembly.
The present preferred embodiment particularly relates to electromagnets comprising essentially cylindrical assemblies of annular coils, aligned about a common axis, but displaced with respect to one another along that axis. Such arrangements are commonly referred to as solenoidal magnets, although they may not be solenoids in the strict sense of the word.
FIGS. 1-4 schematically illustrate certain arrangements of coils bonded to support structures as solenoidal magnets.
FIG. 1 shows a very well known, conventional arrangement in which coils 10 of superconducting wire are wound into annular cavities within a former 12. The structure essentially has 360 degree symmetry about the A-A axis, and also essentially has reflection symmetry about the plane B-B. The former is typically a turned aluminum tube in which annular channels are formed. In other less common variants, the former may be molded, or turned in a composite material such as glass fiber reinforced epoxy resin. In a typical manufacturing process, the coils 10 are impregnated with a hardening material, typically an epoxy resin, which bonds the wire in the coils together. The coils 10 are typically insulated from the former 12 on their radially inner surface (known as the A1 surface); the axially inner surface (known as the B1 surface) and the axially outer surface (known as the B2 surface) using materials to create slip planes between the coil and the former. These dimensions are defined with respect to the magnet center. In an alternative embodiment the coils may be bonded to the support structure on all faces.
Surfaces A1 and A2 are respectively at radii A1, A2 from axis A-A, and surfaces B1 and B2 are respectively at axial displacements B1, B2 from the plane B-B, as shown in FIG. 1. A so called ‘central coil’ is defined with respect to the B-B as symmetry plane as having B1=0 and B2 reflected by the symmetry plan. All other coils can be defined by B1 and B2 which are reflected in the symmetry plan.
FIG. 2 shows an alternative arrangement, in which such a former is not provided. Instead, coils 10 are bonded on their radially outer surface (known as the A2 surface) to a support structure 14, typically in an essentially cylindrical shape. This structure may be manufactured by winding the coils 10 into a mold, winding a filler material such as glass fiber cloth over the radially outer surface of the coils, and impregnating the whole structure with a hardening material such as an epoxy resin. The coils 10 are accordingly bonded to the support structure 14 only by their radially outer (A2) surfaces.
FIG. 3 shows another possibility. Here, coils 10 are wound between support elements 16. The coils 10 are bonded to the support elements 16, for example by a hardening material such as an epoxy resin. The coils 10 are accordingly bonded to the support structure comprising support elements 16 only by their axially inner (B1) and axially outer (B2) surfaces. Such a structure may be formed by temporarily attaching support elements 16 to a winding tube, winding coils 10 onto the winding tube between the support structures, and impregnating the coils 10 with a hardening material such as epoxy resin, which also serves to bond the coils 10 to the support elements 16.
The support elements 16 of FIG. 3 may be annular pieces of aluminum, composite material or any material which has suitable properties of mechanical strength, coefficient of thermal expansion, and density. Suitable materials include metals, typically, aluminum and stainless steel; composite materials such as those known under the Trademarks Tufnol and Durostone; various epoxy resins filled with either glass halls or cloth; or any other combination of materials which have suitable properties of mechanical strength, Young's modulus, and coefficient of thermal expansion.
FIG. 4 shows a partial cut-away view of a variant of the arrangement of FIG. 3, in which annular support elements 16 of FIG. 3 are replaced by support blocks 18, which are spaced circumferentially around the axial faces of the coils.
This structure may be manufactured by a process similar to that described for manufacturing the structure of FIG. 3, but in which spacer blocks (not shown) are positioned between the support blocks 18 to ensure the correct spacing of the support blocks, to support the winding of the coils, and to displace resin during the impregnation process. These spacer blocks may be removed from the structure after resin impregnation.
In this arrangement, the coils 10 are accordingly bonded to the support structure comprising support blocks 18 only by their axial inner (B1) and axial outer (B2) surfaces, and then only at circumferentially spaced locations.
The support blocks 18 of FIG. 4 may be pieces of aluminum, composite material or any material which has suitable properties of mechanical strength, coefficient of thermal expansion and density. Suitable materials include metals, typically, aluminum and stainless steel; or composite materials such as those sold under the trademarks Tufnol and Durostone; various epoxy resins filled with either glass balls or cloth; or any other combination of materials which have suitable properties of mechanical strength, Young's modulus, and coefficient of thermal expansion.
The coils 10 are made up of superconducting wire, which is typically made up of a matrix of NbTi filaments in a copper matrix. The turns of the wire are separated by a very thin layer of electrically insulating material, such as an epoxy resin. However, the thermal expansion coefficient and thermal conductivity of the coil are close to those of copper in the circumferential direction. In the radial and axial directions the thermal expansion coefficient is determined by a combination of the thermal expansion coefficients of the composite layers of wire and resin.
The materials of the support structure—for example aluminum or GRP (glass fiber reinforced plastic)—will have rather different thermal conductivities and thermal expansion coefficients. When the assembly of coils and support structure undergoes an abrupt change of temperature, the coils and the support structure will expand or contract to differing extents, and at differing rates. For materials with a relatively low thermal conductivity, the change in temperature will only take effect slowly, while temperature changes will take effect more rapidly for materials with a higher thermal conductivity. Furthermore, materials with a greater coefficient of thermal expansion will expand or contract as a result in the change of temperature to a greater extent than materials of lower thermal expansion coefficients.
As materials expand or contract with temperature, a value of strain may be defined as the proportion by which a dimension of the material changes. For example, if an object of length d changes length by Δd, the associated strain may be expressed as Δd/d.
The strain values for the different materials will be different even though their temperature changes may be similar.
In any of the coil assemblies described above, the strain in the coils will be different from the strain in the adjacent support structure. This risks damage to the bonded interfaces between coils and support structure because of a shear strain at the bonded interface. The resulting mechanical forces on the coils may risk a movement of the coils when in use, cracking at the interface of the coil bonded to the support structure, and bending of the coil causing stress and internal cracking, which may lead to a quench.
During a quench, the energy stored in the magnetic field of a superconducting magnet is suddenly dissipated into heat within the coils and magnet structure because of a disturbance to the superconducting state, typically caused by heat created by a mechanical interaction with the support structure or internal cracking of the resin within the coils, or over stressing the wire. Many known arrangements provide for the energy to be spread across several coils, following a quench in one coil. However, this will result in rapid heating of the coils, but the support structure bonded to the coils will not heat as quickly. This will result in a difference of surface strain between coil and support structure, risking damage to the bonds between coils and support structure.
In magnet structures such as shown in FIG. 1, with slip planes between the coils and the support structure the coils may be free to move independently of the support structure, and therefore damage between the coil and the former is restricted to stick slip issues. In magnet structures similar to that shown in FIG. 1 with the coils bonded to the former, damage to the bond between the coil and former may lead to a quench.
In magnet structures such as shown in FIG. 2, damage to the bond between coil 10 and support structure 14 may allow the coil some axial movement, which may in turn lead to a quench.
In magnet structures such as shown in FIG. 3 and FIG. 4, damage to the bond between coil 10 and support structure 16, 18 may damage the mechanical integrity of the structure as a whole, which is held together essentially only by bonds between coils and support elements. Damage to the bond may take the form of cracking which may result in the magnet quenching.